1. Field of the Invention
The present invention generally relates to communication systems and in particular to wireless communication systems.
2. Description of the Related Art
As wireless communication systems evolve, there is an increasing need to accommodate wireless communication systems that not only convey (i.e., transmit and/or receive) voice but also allow data information to be conveyed between users of the communication system. The data information is various types of digital information such as text, graphics and other digital information that are typically not time sensitive. Information such as voice or video are time sensitive in that once transmission has commenced there can be no appreciable delay in subsequent transmissions. Any appreciable delay in consecutive transmissions of the time sensitive information causes the information to become unintelligible to a receiving user equipment (i.e., a mobile station). Data information, on the other hand, can tolerate delays in consecutive transmissions and thus can be processed differently from time sensitive signals.
Wireless communication systems, such as systems that comply with the well known 1x-EV-DO (CDMA 2001x-Evolution-Data Optimized) and 1xEV-DV (CDMA2001x-Evolution-Data Voice) standards as well as the High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard can accommodate the conveyance of data information and are hereinafter referred to wireless data systems. A standard is a set of protocols established by standard bodies such as industry groups and/or governmental regulatory bodies. A protocol is generally a set of rules that dictate how communication is to be initiated, maintained and terminated between system equipment and/or user equipment of the communication system. The wireless data systems are structured in substantially the same manner as other wireless communication systems in that they comprise a plurality of base stations located in cells. A cell is a geographical area defined by physical boundaries. Each cell has base station equipment (or cell site) that services user equipment (UE) located in that cell. The UE is being serviced when the base station equipment provides the UE with appropriate amounts of various resources (e.g., power, bandwidth) to enable the UE to convey adequately information to other users or other system equipment. Base station equipment is generally system equipment comprising communication equipment (e.g., radio transmitters, receivers, processing equipment) owned, controlled and operated by system providers. System providers are entities such as local telephone companies, long distance telephone companies, Internet Service Providers (ISP) and other communication service providers. Examples of UE include cellular telephones, pagers and wireless personal computers.
A UE receives information from base station equipment over a downlink and transmits information to base station equipment over an uplink. The uplink comprises at least one traffic channel and at least one signaling channel. Similarly, the downlink comprises at least one signaling channel and at least one traffic channel. The traffic channel is a communication channel over which user information or traffic channel information (e.g., voice, video, data) is conveyed between UEs and system equipment of the communication system. The signaling channels are communication channels used by the system to manage, and otherwise control the operation of communication channels of the communication system. In a communication system that is to comply with the UMTS HSDPA developing standard, a key downlink signaling channel is called the High Speed Shared Control CHannel (HS-SCCH) and a key uplink signaling channel is called the High Speed Dedicated Physical Control CHannel (HS-DPCCH). The signaling information conveyed over the HS-SCCH and the HS-DPCCH is referred to as control information.
In communication systems to comply with the UMTS HSDPA standard (being developed), control information sent over the HS-SCCH (downlink signaling channel) and the HS-DPCCH (uplink signaling channel) are transmitted during a time interval called a Transmit Time Interval (TTI). The TTI is divided into three equal slots. Typically, for UMTS systems the TTI is 2 ms in length whereby each slot is ⅔ ms in length. For the HS-DPCCH, each slot contains 10 bits of information. The information contained in the HS-DPCCH slots are used by the UE to feedback measured information about the downlink channel and success of previous transmissions to adequately manage the amount of power to be allocated to downlink traffic signals transmitted to a UE. Other information contained in the HS-DPCCH slots are proposed to be used by the system to adequately manage the amount of power allocated to the uplink control and/or data and/or other traffic signals transmitted by a UE. In general these information are used to properly manage communications between the UE and the base station servicing the UE. In particular, the first slot contains ACK/NACK information which is acknowledgment information transmitted by the UE to its servicing base station ACKnowledging or Not ACKnowledging the proper reception of traffic channel information from the base station. The remaining two slots, which contain 5 bits of information coded to 20 channel bits (10 bits/slot) are used for CQI (Channel Quality Indicator) information. CQI is also known Channel Quality Feedback (CQF) information. The CQF is information transmitted by the UE to the base station to indicate the relative quality of the signals received by the UE over the downlink.
The servicing base station modifies its transmitted signals based on the CQF information it receives from the UE; in this manner the quality of the downlink signals is adequately maintained to enable the UE to properly receive downlink signals from the servicing base station. In UMTS, 1 bit is actually used for the ACK/NACK and 5 bits are actually used for the CQF. Thus, there are 32 levels of downlink channel quality where level 0 indicates a relatively very low quality signal being received from the base station and level 31 indicates a relatively very high quality signals being received from the base station over the downlink. The CQF thus helps the base station control the quality of the downlink signals. The 5 CQF bits are channel coded into 20 bits to protect the CQF from becoming erroneous when it is propagating through the HS-DPCCH. Channel coding is a well known technique of introducing redundancy in a block of data to protect the data from error causing noise in a communication channel through which the data is transmitted.
To control the quality of the uplink signals, the base station uses another (legacy) signaling channel called the UL-DPCCH (Uplink Dedicated Physical Control Channel) that contains a pilot channel. The pilot channel contains a pilot signal that is periodically transmitted by the UE being serviced by the base station; this pilot signal is hereinafter referred to as the ‘legacy pilot.’ Based on the power of the legacy pilot signal received by the base station over the uplink pilot channel, the base station sends a legacy pilot power control signal over another “legacy” downlink control channel called the DL-DPCCH (Downlink Dedicated Physical Control Channel) typically instructing or commanding the UE to increase, or decrease the power of its signals being transmitted over the uplink control/traffic channel. The legacy pilot power control command signal is sent during each of the three slots of the HS-DPCCH TTI. The legacy pilot signal is thus used, inter alia, as a reference signal for controlling the proper transmission power of uplink signals. It should be noted that the legacy pilot signal is used for other purposes which are not discussed herein as these purposes are not within the scope of this invention.
The UE sometimes enters into a state called a handoff in which the UE is being serviced by more than one base station simultaneously. Several base stations receive the legacy pilot signal and transmit legacy pilot power control signals to the UE. At this point the UE is transmitting information (signaling and traffic information) to several base stations simultaneously. The traffic channel information received by each of the base stations are transferred to system processing equipment which combine the various received traffic information to obtain the correct block of information that was sent. However, each of the base station is transmitting a legacy pilot power control signal command that may be contradictory to other legacy pilot power control signal commands from the other base stations in handoff with the UE. For example one base station may send an ‘increase power command’ while another base station sends a ‘decrease power command.’ To overcome this contradiction in legacy pilot control signal commands, the system typically adopts an “Or of the Down” algorithm. In the “Or of the Down” algorithm, the UE will decrease its transmit power if any of the base stations with which it is in handoff transmits a legacy pilot power control command indicating ‘decrease power.’ The UE will increase its transmit power only if all of the base station with which it is in handoff transmit an ‘increase power’ command.
While in handoff with several base stations, the UE may also be receiving data information from another base station (e.g. HSDPA serving base station) over a data traffic channel (e.g. HS-DSCH or High Speed Downlink Shared Channel). At this point in the development of wireless data communication systems (i.e., systems that convey voice and data simultaneously), there is no handoff procedure for packet data traffic (e.g. HSDPA). For example, only one HSDPA serving base station sends packet data at a time on the HS-DSCH traffic channel to the UE. In turn, the UE sends control information and/or packet data to only one base station (e.g. HSDPA) at a time, for example, on the HS-DPCCH . However, the power allocated to the uplink pilot signal does have a direct proportional effect on the power of the control signals being sent over the HS-DPCCH because it is the pilot signal that is used to control the power allocated to the entire uplink. Therefore, a UE in handoff may be decreasing its transmit power due to commands processed in accordance with the “OR of the Down” algorithm while at the same time the base station (HSDPA) which is receiving control information/packet data from the UE needs the transmit power to be increased. Therefore, a contradictory situation can exist while a UE, which is capable of transmitting/receiving both data and voice, is in handoff.
One proposed technique that attempts to address the contradictory problem is to create another pilot signal called the High Speed pilot (HS pilot signal) signal that is used to control uplink transmit power of HSDPA related control information and/or other packet data signals independently of the transmit power of voice, circuit data or other such legacy signals. The HSDPA base station communicating with the UE and receiving the HS pilot signal will transmit HS pilot power control commands signals instructing or commanding the UE to either increase, maintain or decrease its transmit power for the HSDPA related control and/or other packet data signals. The HS pilot power control commands are transmitted during one of the slots of the downlink TTI (DL-DPCCH) replacing one of the legacy pilot control signals. Thus, with this technique, an HS pilot power control signal is transmitted every third slot of the HS-DPCCH TTI while the legacy pilot control signal is transmitted during two slots for every TTI. This technique is not very invasive because only one slot of the legacy pilot control signal is ‘stolen’ for the use of the HS pilot signal. The power of the legacy pilot signal and thus the power of the uplink for voice, circuit data and other such legacy signals will still be controlled but at a slightly lesser rate.
In the proposed technique, the HS pilot signal is inserted in part of the first slot of the HS-DPCCH. The first slot, as previously discussed, contains the ACK/NACK which is represented by 1 bit of information that is coded to 10 bits. Typically a “1” bit represents an ACK response and a “0” bit represents a NACK response. The coding done is to duplicate the response so that an ACK becomes “1111111111” (10 “1” bits) while a NACK becomes “0000000000” (i.e., 10 “0” bits). It is well known that the best coding that can be done for a one bit piece of information is to simply replicate that one bit. Further, this type of coding is not a very robust type of coding in that it is relatively quite vulnerable to noise in the channel within which it is transmitted. Yet further, the required bit error rate for the ACK/NACK slot is typically on the order of 10−4 or less. In other words, for every 10,000 bits of ACK/NACK information that is sent only one erroneous bit is allowed. The bit error rate requirement for the ACK/NACK is relatively quite stringent because the ACK/NACK signal is crucial in managing the efficiency (in terms of power and bandwidth) of the system downlink. Thus, the usage of some of the ACK/NACK bits for the HS pilot signal will most likely damage the reliability of the ACK/NACK information received by a base station and reduce the base station's ability to efficiently manage the downlink resources.